KIEV, UKRAINE—Russia’s military staged a provocative new act of aggression on Saturday, occupying a gas-pumping station and village on a narrow strip of Ukrainian land near the Crimean Peninsula and prompting Kiev’s Ministry of Foreign Affairs to denounce “a military invasion by Russia.”

The incident marks a fresh escalation in already-high tensions as well as the first face-to-face standoff between the Ukrainian and Russian militaries outside the Crimean Peninsula, suggesting Moscow is testing the will of Kiev amid fears of further Russian incursions in eastern and southern Ukraine. The move comes on the eve of a vote in Crimea on whether the residents of the peninsula want to break away from Ukraine and join Russia.

A force of Russian troops in four helicopter gunships and three armored combat vehicles descended on a pumping station near the village of Strilkove around 1:30 p.m. local time, according to Ukrainian officials.

The Russians said they had seized the pumping station out of fears it would be targeted by “terrorists,” according to a Ukrainian defense ministry official who declined to be named.

Oleg Slobodyan, a spokesman for the Ukrainian boarder guard, said Saturday evening that 120 Russian soldiers were still occupying the pumping station, and had not agreed to Ukrainian demands to leave. No shots had been fired, he said, but added that Ukrainian military forces had mobilized to just outside the city of Henichesk, putting themselves between the Russian force and the Ukrainian mainland.

Initially, Ukraine’s Defence Ministry said the Russians had appeared to leave the area after a period of negotiation, but a Foreign Ministry statement suggested the Russians remained in place.

“Ukraine Foreign Ministry declares the military invasion by Russia and demands the Russian side to immediately withdraw its military forces from the territory of Ukraine,” the ministry said in a statement. “Ukraine reserves the right to use all necessary measures to stop the military invasion by Russia.”

The standoff, which apparently occurred about 130 kilometres from Crimea, could mark a significant escalation in a conflict that shows no sign of easing a day before the Crimeans vote.

The allegations by Ukrainian authorities came as tens of thousands of antiwar demonstrators turned out in Moscow to protest Russia’s intervention in Ukraine, marking the largest political rally in the Russian capital in more than a year.

Earlier on Saturday, Ukrainian officials reported a shootout between pro-Russian and pro-Ukrainian demonstrators that took place overnight Friday in the eastern Ukrainian city of Kharkiv.

Two people were left dead and dozens injured after a group of Russian separatists approached offices being used by pro-Ukrainian activists, according to Tatiana Gruzinskaya, spokeswoman for the city’s mayor.

The Russian Foreign Ministry issued a menacing statement Saturday in response to the violence. “There have been many appeals for Russia to protect peaceful civilians,” the statement said. “These appeals will be considered.”

That warning, which follows other, similar threats, is likely to add to fears that Russia will expand its intervention beyond Crimea and into eastern Ukraine. While Russia has blamed such clashes on right-wing Ukrainians and on the new government, Ukrainian officials placed responsibility squarely on Russia’s shoulders.

Speaking from the Ukrainian capital of Kiev, interim President Oleksandr Turchynov said it was “Russian agents” who were “causing people to be murdered.”

As Turchynov spoke, demonstrators in Moscow showed their solidarity with Ukraine by carrying Russian, Ukrainian and European Union flags as they walked from Pushkin Square to Sakharov Prospekt, which they filled. Participants also protested against increasing pressure the government has been bringing to bear on Russian news organizations.

One indication of the state of the media in Russia was in the reported turnout for the demonstration. Russian wire services quoted police as saying 3,000 people had taken part — but that was nowhere near the truth. Dozhd TV, an independent channel that earned the Kremlin’s ire and now faces closure, put the number at 50,000.

“No blood. No tears. We don’t need any more,” read one sign.

“We are Russia. Not Putin,” read another.

“The result of this is to show Ukrainian citizens our solidarity, so they will see there is another Russia, a Russia that doesn’t want war,” said Maria Lobanova, 30, who had come to the rally with her father, husband and two sons, ages 4 and 1. “I don’t understand why Europe and the United States talk about sanctions so much but don’t do anything about them.”

The shootout in Kharkiv left two people dead, acting Interior Minister Arsen Avakov wrote on his Facebook page Saturday. It was not yet clear whether the fatalities were pro-Russian or pro-Ukrainian, and activists on both sides were arrested, officials said.

On Saturday in Kiev, Ukrainian Foreign Minister Andriy Deshchytsya called on Russia to “stop interfering” in eastern Ukraine, saying that authorities here could handle issues of law and order in the region themselves. He described the threatening statements from Russia’s Foreign Ministry as an attempt to “provoke” and escalation of the tense situation in the east.

Deshchytsya called for U.N. monitors to fan out across southern and eastern Ukraine to independently assess the situation on the ground.

He also reiterated Kiev’s position that Sunday’s referendum in Crimea would be “illegal” and would not be recognized by the international community. He emphasized the new government’s desire to avoid any military conflict and referred to the current stand off as more of a “diplomatic war.” But he added that he would travel to Brussels on Monday to meet with NATO’s secretary general to discuss “military and technical cooperation.”

Russia on Saturday blocked passage of a U.N. Security Council resolution that would have declared that Sunday’s referendum “can have no validity, and cannot form the basis for any alteration of the status of Crimea.” The veto was expected, but in a possible sign of Russian isolation, Moscow’s close ally China opted to abstain rather than join Moscow in vetoing the measure. The other 13 Security Council members supported the measure.

U.S. Secretary of State John Kerry on Friday warned against a “back-door annexation” of Crimea by Russia. But he conceded that six hours of talks in London with Russia’s top diplomat neither stopped Sunday’s vote nor opened a new diplomatic path for Moscow to step back from the Cold War-tinged standoff.

The United States and other nations have been dangling a diplomatic solution for Russia, tacitly acknowledging that the referendum would produce a pro-Russia outcome while suggesting that Russia could avert further escalation by leaving Crimea’s status vague.

Kerry said Russian Foreign Minister Sergei Lavrov had made it clear that Russian President Vladimir Putin was not prepared to take that step.

“We don’t have a common vision of the situation” after the crisis talks, Lavrov said.

Lavrov said Russia will “respect” the results of the plebiscite, and it was clear from Kerry’s tone that the United States fears full annexation. When pressed about whether Russia would annex Crimea after the vote, Lavrov said, “There are no what-ifs in politics.”

A new poll suggests the Crimean vote to join Russia will be overwhelming.

In a GfK poll of 600 residents taken Thursday and Friday, 70 per cent said they will vote to become part of Russia, while 11 per cent said they will vote to restore Crimea’s status as part of Ukraine. If they were offered more options, they told pollsters, 19 per cent would vote for independence. But a majority, 54 per cent, would still favour becoming joining Russia.

The vote will be held under the eyes of Russian troops, who effectively took control of Crimea late last month after protesters overthrew the Ukrainian government. A Ukrainian Defence Ministry official said a column of about 50 armored vehicles from the Russian Federation were observed Friday night moving from the city of Feodosia to Dzhankoi in northern Crimea. Ukrainian military spokesman Vladislav Seleznyov said more Russian troops have moved in to occupy Ukrainian military bases that had been abandoned long before the crisis began.

Moscow’s tightening grip on Crimea and the gathering of Russian troops along the countries’ border have unnerved Ukrainians and left the country’s fledgling government concerned about further Russian military action. Kerry said the United States was “deeply concerned” about those deployments.

If Russia does move to annex Crimea after Sunday’s vote, it could trigger the harshest sanctions levied against Moscow by the United States and European powers since the Cold War.

The European Union is expected to impose travel bans and asset freezes Monday on Russians accused of complicity in Moscow’s military incursion and the intimidation of Crimea. The E.U. on Friday identified more than 120 individuals as potential sanctions targets.

The White House announced Friday that Vice President Joe Biden will travel to Poland and Lithuania next week to discuss Ukraine and other issues with regional leaders.

In the Crimean capital of Simferopol, officials on Saturday tried to put a professional and orderly face on arrangements for the referendum, with decidedly mixed success. Hundreds of journalists waited for hours outside the Crimean parliament building for press credentials, facing double lines of Cossack guards who shoved and shouted at several who tried to slip past. Finally, everyone was told to go to another press center for the polling day passes.

At a midday news conference, senior Crimean election official Mikhail Malyshev, provided the first detailed figures for the referendum process. Denying widespread rumors that tens of thousands of extra ballots had been prepared, he said only 1,512,000 had been printed in accordance with the latest lists of registered voters. He said that 98 per cent of polling places would be the same as in previous elections, many of them in schools.

Asked about rumors that voting cards had been sent to dead people, he responded, “Unfortunately, my mother had died and she got one, too. These things happen, but it won’t affect the results.” He said turnout estimates would be released at various times Sunday but did not say when or where referendum results would be announced.

Malyshev said that 622 journalists, 123 foreign observers from 23 countries and 1,240 observers from Crimean organizations had been registered for the referendum. He did not list the countries sending observers but said they include members of parliaments from many European nations. Delegates from the Organization for Security and Cooperation in Europe have repeatedly been denied access to Crimea in the past week.

Vasilyev Maxim, an official from the Russian city of Kursk, said he drove to Crimea with thousands of Russian flags purchased with $10,000 of his own money to distribute ahead of the vote. “In 30 years, the history books will say Putin took back Crimea to rebuild our country,” he said. “And no one will remember Kerry or that Obama had anything to do with the situation.”